Unified Inch Screw Threads ( UN and UNR Threads ) :
These threads are used for general fastening purpose and available as Unified Coarse thread series (UNC/UNRC), fine thread series (UNF/UNRF), extra-fine series (UNEF/UNREF).
The most commonly used classes of theads are 2A & 3A for external threads and 2B & 3B for internal threads.

The Stub Acme screw threads are generally confined to those unusual applications like transmission of power and motion where a coarse pitch thread of shallow depth is required due to mechanical or metallurgical considerations.

Round Threads :
These threads are also known as Knuckle threads and are insensitive to dirt and damage due to their round shape and are used in fastening screw threads in clutch of railway cars and for large valves and gates, for bottle caps etc.